chore: Bump org.springframework.boot from 3.5.5 to 4.0.5 in /backend

Bumps [org.springframework.boot](https://github.com/spring-projects/spring-boot) from 3.5.5 to 4.0.5.
- [Release notes](https://github.com/spring-projects/spring-boot/releases)
- [Commits](spring-projects/spring-boot@v3.5.5...v4.0.5)

---
updated-dependencies:
- dependency-name: org.springframework.boot
  dependency-version: 4.0.5
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

backend/build.gradle

@@ -1,7 +1,7 @@
 plugins {
     id "java"
     id "idea"
-    id "org.springframework.boot" version "3.5.13"
+    id "org.springframework.boot" version "4.0.5"
     id "io.spring.dependency-management" version "1.1.7"
     id "io.freefair.lombok" version "9.2.0"
     id "com.github.spotbugs" version "6.5.0"